realtek: remove RTL839x path in RTL838x multicast

realtek: remove RTL839x path in RTL838x multicast

The multicast setup function rtl838x_eth_set_multicast_list() checks if
the current SoC is a RTL839x family device. However, the function is
only included in the RTL838x ops table, so this path should never be
taken, making this dead code. rtl839x_eth_set_multicast_list() is
already present in the RTL839x ops table, so it should be safe to remove
this branch.

While touching the code, also re-sort the functions to match sorting
elsewhere, with rtl838x coming before rtl839x.

Signed-off-by: Sander Vanheule <sander@svanheule.net>